The journey to Fukushima Daiichi begins at the border of the 12-mile exclusion zone that surrounds the ruined nuclear complex, beyond which life has frozen in time. Weeds reclaim the gardens of empty homes along a route that emptied on a bitterly cold night almost a year ago. Shop signs hang unrepaired from the huge quake that rattled this area on 11 March, triggering the meltdown of three reactors and a series of explosions that showered the area with contamination. Cars wait outside supermarkets where their owners left them in Tomioka, Okuma and Futaba – once neat, bustling towns. Even birds have deserted this area, if recent research is to be believed.

The reason is signalled by a symphony of beeping noises from dosimeters on our bus. As we drive through a police checkpoint and into the town of Tomioka, about 15km from the plant, the radioactivity climbs steadily, hitting 15 microsieverts per hour at the main gate to the nuclear complex. At the other end of the plant, where the gaping buildings of its three most damaged reactors face the Pacific Ocean, the radiation level is 100 times this high, making it still too dangerous to work there.

Inside the plant's emergency co-ordination building, the air is filled with the sound of humming filters labouring to keep the contamination out. Hundreds of people work here, many sleeping in makeshift beds. Workers in radiation suits and full-face masks wander in and out. A large digital clock showing the current radiation reading inside the building dominates the wall of the central control room, where officials from operator Tokyo Electric Power (Tepco) huddle around computers.

"Our main challenge now is to remove the nuclear fuel from the reactors," explains Takeshi Takahashi in his first interview since he took over as plant manager two months ago. "It's a technically very difficult problem, but we cannot hurry." His predecessor Masao Yoshida was forced to quit in December after being diagnosed with cancer – unrelated to his work, insists Tepco.

Mr Takahashi looks exhausted but says he is satisfied with the progress being made in bringing the plant to "a state of cold shutdown", meaning radiation releases are under control and the temperature of its nuclear fuel is consistently below boiling point.

The term is considered controversial. Engineers have only a rough idea of where exactly the melted fuel lies inside the damaged reactors, or of its exact state. The fuel is being kept cool by thousands of gallons of water that Tepco pumps on to it every day and which it is struggling to decontaminate. Engineers are frantically working to build more water tanks – on a ridge about 65ft from the reactors is a field of 1,000-ton water tanks. A crew is levelling land to make way for more.

We are told to wear our full-face masks for the climax of the visit – a tour of the six reactors. Every inch of our bodies is covered and even in the sub-zero temperatures of Fukushima in February, it is unbearably hot. Thousands of men worked through last year's summer heat of over 30C in this protective gear, struggling to clear debris and bring water to the reactors. "They were dropping like files in the heat," said one worker. "But they just had to keep going."

"The worst time was when the radiation was 250 milisieverts [per year – the maximum, temporary government limit] and we couldn't find people to do the work," explains Kazuhiro Sakamoto, an onsite subcontractor. "We could only work in two-minute bursts, when we were extracting caesium from contaminated water."

Some of that work is clear on site. The concrete building housing Reactor One, blown apart in the first explosion on 12 March, is now completely covered with a tarpaulin to contain its radioactivity. As our bus drives past the building, the beeping dosimeters climb to 100 microsieverts an hour. But as the most badly damaged Reactor Three looms into sight, its mess of tangled metal and steel gives off a startling reading of 1,500 microsieverts. Its cargo of lethal fuel includes plutonium and the roof of the building housing the reactor was blown off in the second explosion. "It's still too dangerous for workers to enter Reactor Three," says engineer Yasuki Hibi. […]